Chemical recovery of waste liquors



INVENTOR m'eij May 19, 1964 sjA. GUERRIERI CHEMICAL RECOVERY OF WASTELIQUORS Filed May 10. 1961 Z'oSlack Salvatore J1. filler AG E N T ya. aw a a W mama Z 5 w, r m a w Tam, W2 1 5Z1 V m, 2 M w mm w m a H A a? z wM; 4% 2 w 1 o 5 a W 6 M m 4 sg WW 0 a 0% *F/ 7 5 6 ml 2 Al M t; w m Jkkwng d M w M i United States Patent 3,133,789 CHEMHIAL REQQVERY 6FWASTE LIQUQRS Salvatore A. Guerrieri, Scarsdaie, N.Y., assignor to TheLummus Company, New York, N.Y., a corporation of Delaware Filed May 16,1961, Ser. No. 109,128 11 Claims. ((31. 2348) The present inventionrelates to a process for recovering chemicals from the waste liquorsproduced in the wood pulping industry. In its more specific aspects,this invention relates to a continuous process for the preparation ofdelignification liquors from sodium base waste sulfite liquors for reusein the pulping processes.

It is well known that most paper and paper products are manufacturedfrom wood and/ or other cellulose materials which have been convertedinto pulp. Pulp may be prepared by several processes includingmechanical, chemical and semi-chemical processes. Mechanical processesare based upon the physical reduction of cellulosic material to afibrous state whereas the chemical processes, such as the acid sulfite,sulfate and soda processes, are based upon the chemical removal of theligneous compounds contained in the cellulosic material. While chemicaland mechanical processes have been widely utilized, it has only been inrecent years that semi-chemical processes have achieved substantialimportance, even though such processes have a substantially higher pulpyield. Semi-chemical processes normally comprise the steps of reactingthe cellulosic material with a chemical liquor to partially remove theligneous compounds and thereafter subjecting the partially delignifiedcellulosic material to a mechanical process to complete fiberization.

A residual waste liquor from a sodium sulfite or sodium bisulfitedelignification contains sulfur dioxide bearing compounds of sodium,principally as ligno-sulfonic compounds or other organic sulfoniccompounds formed by the action of the cooking liquor on the organicsubstances present in the ligneous cellulosic material, together withsome residual sulfite compound or compounds. It also ordinarily containsother organic compounds derived from the cellulosic material or formedby the action of the cooking liquor on constituents of such materialincluding, for example, sugars or other carbohydrates, and variousorganic acids, such as formic or acetic acid or the salts thereof.

The present invention is applicable to the treatment of the residualliquor obtained from delignifying processes utilizing various types ofsodium base sulfite chemicals such as the sodium base acid sulfiteprocesses, in which the cooking liquor contains sodium acid sulfite(NaHSO and generally free sulfurous acid, whereas the residual liquorcontains some sodium acid sulfite together with ligno-sulfonic compoundsand other organic compounds as mentioned hereinabove, and the neutralsodium base sulfiite processes in which the cooking liquor containssodium sulfite (Na SO and one or more alkaline compounds of sodium suchas sodium bicarbonate and/or sodium carbonate, whereas the residualliquor contains sodium sulfite with some of the other alkaline sodiumcompounds together with ligno-sulfonic compounds and other organiccompounds as mentioned hereinbefore.

It is generally known that the products obtained from the combustion ofa concentrated waste sulfite liquor contains in addition to sodiumcarbonate substantial quantities of sodium sulfide which should beconverted if these products are to be treated to regenerate a sodiumsulfite cooking liquor. This conversion step should be performed in sucha manner as to minimize the formation of the undesirable sodiumcompounds, such as sodium thiosulfate and sodium polysulfides which maybe formed in the presence of residual sodium sulfide. Sodium thiosulfateand polysulfides cause great difiiculties in subsequent digestioncycles, even though present in relatively small amounts, since sodiumsulfide, sodium thiosulfate and sodium polysulfides in the presence ofsulfur dioxide form free sulfur which has a deleterious effect on thepulp product obtained from an acid delignification, particularly insubsequent bleaching operations.

It is known to concentrate waste liquors from delignification processesused in the pulping industry, to burn and to smelt such liquor in aprimary recovery unit, and to dissolve the smelt prior to subjecting itto the action of other chemicals. As a result of the generalcomplexities of such processes, notwithstanding the simple chemistryinvolved, the prior art recovery and conversion plants are relativelycomplicated and expensive.

It is a principal object of this invention to provide an improvedprocess for treating waste delignifying liquors so as to permit reuse ofthe chemicals therein in subsequent delignification cycles.

Another object of this invention is to provide a novel method oftreating the smelt formed by burning such waste liquors to convert thesodium sulfide content thereof to sodium carbonate.

These and other objects, and a fuller understanding of the invention maybe had by referring to the following description taken in conjunctionwith the accompanying drawing, in which the figure is a schematic flowdiagram illustrating a preferred embodiment of my in vention.

I propose to treat in a reactor the smelt (formed by burning theconcentrated black liquor) with carbon dioxide and water vapor at atemperature above the melting point of sodium carbonate, i.e. 851 C.While the smelt withdrawn from a recovery furnace is generally in themolten state, the smelt may be introduced into the reactor in the solidstate and will pass into the molten state during the conversion of thesodium sulfide to sodium carbonate at the temperature maintained in thereactor. However, care must be taken not to cause the molten smeltobtained from the furnace to be contacted with air, and further, thesmelt in the solid state must be at a temperature sufficient whereby theheat of reaction will maintain the converter at appropriate reactionconditions. It is important that the temperature in the reactor bemaintained above the melting point of sodium carbonate to avoid cloggingup the reactor. Preferably, the temperature in the reactor is maintainedsubstantially above the melting point of sodium carbonate so as toconvert substantially all of the sodium sulfide in the smelt to sodiumcarbonate. Further, with high conversion temperatures, the rate ofreaction will be higher, thus resulting in smaller sized equipment. Thethus treated smelt is thereafter passed through a series of processingsteps whereby cooking liquors are formed for use in delignifyingcellulosic materials.

Referring now to the drawing, and more particularly to FIGURE 1, thepreferred embodiment comprises the following principal components,namely a primary recovery furnace 10 for burning the concentrated wasteor black liquor, a converter 11 in which sodium sulfide in the moltensmelt received from the furnace is subjected to chemical reactions, adissolver 12 and a clarifier 13 adapted to recover sodium carbonate fromthe smelt leaving the converter, and carbon dioxide and sulfur dioxiderecovery units 15 and 17, respectively.

Concentrated black liquor is fed into the recovery furnace 10 throughline 19. In the furnace lit the black liquor is burned to form gaseousproducts and a smelt primarily containing sodium carbonate and sodiumsulfide. The temperature in the furnace 10 is maintained at a value atwhich the concentrated black liquor will burn in the presence of acombustion supporting medium.

Combustion air and make-up sulfur for the system are admitted to thefurnace through lines 21 and 21, respectively. The gaseous productsformed in the converter 11 consisting primarily of hydrogen sulfide,carbon dioxide and water vapors may also be passed into the furnace 10via line 22 wherein the hydrogen sulfide is converted to sulfur dioxide.The gaseous products leaving the furnace 10 are passed through line 23to sulfur dioxide and carbon dioxide recovery units 15 and 17 which willbe described in detail subsequently. The combustion gases in line 23consist primarily of carbon dioxide, water vapor, sulfur dioxide, andnitrogen.

In accordance with one embodiment of my invention, the smelt in theconverter is reacted with carbon dioxide and water vapor introducedthrough line 24 and which is heated prior to introduction in a suitablemanner (not shown). The conversion of sodium sulfide to sodium carbonatemay be represented by the following reactions:

The conversion of the sodium sulfide to sodium carbonate is representedby the overall Equation 2 above. It will be appreciated that the aboveequations represent the primary reactions taking place. It is obviousthat insignificant side reactions will also take place, such as, forexample, the formation of minor amounts of sodium sulfate. The converter11 is maintained at a temperature above the melting point of sodiumcarbonate (i.e. 851 C.) and preferably at a substantially highertemperature above the melting point for efficient and effectiveoperation.

The gaseous products formed in the converter 11 are passed into thefurnace 10 through line 22 as previously stated. Alternatively, thesegases may be vented to an auxiliary heat exchange unit to undergocombustion and heat recovery.

The thus treated smelt mainly consisting of sodium carbonate is passedto dissolver 12 through line 25 wherein water is added to form asolution which is known in the art as green liquor. The green liquor(sodium carbonate solution) is withdrawn from dissolver 12 through line26 and is passed to the clarifier 13.

The sodium carbonate solution in clarifier 13 may be passed partiallythrough line 27 directly to the second stage of a two-stagedelignification cycle, partially through lines 28 and 29 to the carbondioxide make-up unit 17, and partially to a sulfite reactor 16 via lines30 and 31. The use of the sodium carbonate solution in the latter twoinstances will be presently described in detail.

Although only one converter 11 has been shown, it is obvious that eithera plurality of converters or a multistage converter may be utilized.Similarly, it is obvious that the carbon dioxide and water vaporintroduced into the converter 11 which is shown entering through line 24may enter via separate lines.

The carbon dioxide and the steam used in the converter 11 may besupplied from any suitable source. For purposes of economy it ispreferable to supply these reactants from sources within the instantsystem or from related sources within the plant. For example, gaseouscarbon dioxide is withdrawn from the reactor 16, a reboiler 18, and ablow gas scrubber 14 and may be passed to converter 11 through lines 32,33, and 34, respectively. Blow gas primarily containing carbon dioxideand sulfur dioxide is introduced into the scrubber 14 from the digesters(not shown). In the scrubber 14, a sodium sulfite solution is passed incounter-current relation to the blow gas whereby the sulfur dioxidecontained therein reacts with a portion of sodium sulfite to form sodiumbisulfite in accordance with Equation 3.

The sodium sulfite solution is introduced into scrubber 14 through line35 and is obtained from reactor 16. The sodium sulfite-sodium bisulfitesolution formed in the scrubber 14 is withdrawn through line 36 and ispassed to sulfur dioxide recovery tower 15 through line 37.

The combustion products leaving the recovery furnace 10 may be vented tothe atmosphere through a stack (not shown). However, it is preferablefrom an economic stand-point to pass these gases through variousprocessing units to recover the sulfur dioxide and/ or carbon dioxidecontents thereof.

In the sulfur dioxide recovery unit 15, the sulfur dioxide in thecombustion gas flowing in the line 23 is removed by passing thecombustion gases counter-current to a sodium sulfite-sodium bisulfitesolution in accordance with Equation 3 above. A portion of the sodiumbisulfite solution formed is passed to a digester in another part of theplant through line 38, with the remaining portion being passed to thereactor 16 through lines 39 and 31.

In the sulfite reactor 16, the sodium bisulfite solution is contactedwith the sodium carbonate solution from clarifier 13 to form a sodiumsulfite solution in accordance with the following equation:

The sodium sulfite solution is transferred through line 35 to scrubber14 and/or through line 37 to the recovery unit 15 for use in recoveringthe sulfur dioxide content of the furnace off-gas according to Equation3 above. The carbon dioxide produced in reactor 16 is passed via lines32 and 24 to converter 11.

After the combustion gases from the furnace 10 have passed through thesulfur dioxide recovery unit 15, they are fed to the carbon dioxidemake-up unit 17 prior to being exhausted to the atmosphere. In thecarbon dioxide make-up or recovery unit 17, the combustion gases arepassed in counter-current relation to a sodium carbonate solutionentering the top of the unit through lines 28 and 29. Carbon dioxide isremoved from the combustion gases by the formation of bicarbonateaccording to Equation 5 below.

The sodium bicarbonate solution is removed from the unit 17 through line40 and passed to reboiler 18 wherein the solution is heated to formcarbon dioxide and sodium carbonate in accordance with the reversereaction shown by Equation 5. The sodium carbonate solution is passed toa digester as a second-stage cooking liquor through line 41 or isrecycled to the unit 17 through line 29. The steam and carbon dioxidewhich are formed are sent to the converter 11 through lines 33 and 24.

Although the sulfur dioxide recovery unit 15 and the carbon dioxiderecovery unit 17 are shown as superimposed, it is obvious that theseunits may be separated.

The feasibility of the process is verified by the results of thelaboratory scale experiment described below.

A mixture of 50% sodium carbonate and 50% sodium sulfide was placed in acylindrical stainless steel cylinder about 1% inches in diameter and 8to 10 inches long and this was placed in an electrical furnace. Heat wasapplied and when a temperature of about 1500 F. was reached, the mixturewas a molten fluid mass. At this point, a mixture of CO and water vaporwas introduced through a tube leading to the bottom of the cylindricalvessel and H 8 was driven off. Since this reaction was carried out inthe open, the hydrogen sulfide formed, burned to sulfur dioxide. As thereaction proceeded and the ratio of sodium sulfide to sodium carbonatedecreased, the melting point of the mixture increased and a cake ofessentially sodium carbonate began to form in the tube. After some 15 or20 minutes the resulting cake was so extensive that gas fiow ceased andthe experiment was discontinued. The resulting product was cooled,dissolved in water and analyzed for sodium sulfide and, sodium sulfidecontent was found to be of the order of magnitude of 3% indicating aconversion of approximately 90%.

The above described process may be modified by supplying air or oxygen)to the converter 11 through the optional air inlet 41. In such a case,the hydrogen sulfide formed in the converter 11 will be oxidizedprimarily to sulfur dioxide in the converter 11 rather than in thefurnace 10. The reaction will proceed according to the followingequation:

It is also within the scope of the instant invention to attain theobjectives thereof without passing carbon dioxide into direct contactwith the molten carbonate and sodium sulfide in converter 11. In suchcase, the reaction in converter 11 would proceed according toEquation 1. The resultant reaction products may be used directly or maybe carbonized by adding a carbonating agent, either carbon dioxide orcarbonic acid at a subsequent point in the system, for example, at thedissolver 12. Reaction (1a) above would then take place with a resultantformation of sodium carbonate.

In a further modification, only a carbonating agent is added to the meltin the converter 11.

For the sake of simplifying the description, various elements of aconventional nature, such as control valves, instruments, and the like,have been omitted from the foregoing detailed description of theinvention.

Obviously, many modifications and variations of the invention ashereinabove set forth may be made without departing from the spirit andscope thereof and, therefore, only such limitations should be imposed asare indicated in the appended claims.

I claim:

1. In a process for treating sodium base waste liquors obtained from awood pulping process, the steps of burning said Waste liquors to form asmelt containing sodium sulfide, contacting the resulting smelt withcarbon dioxide and steam to convert sodium sulfide to sodium carbonate,and maintaining the temperature of said conversion above 851 C.

2. In a process for recovering a solution containing sodium carbonatesubstantially free from sodium sulfide from a sodium base waste liquorfrom a wood pulping process, the steps of burning said waste liquor toobtain a smelt containing sodium sulfide, contacting said smelt withcarbon dioxide and steam to convert sodium sulfide to sodium carbonate,maintaining the temperature of said conversion above 851 C., removingthe gaseous products formed, and dissolving the remaining products toform said solution.

3. In a process for treating sodium base waste liquors from a woodpulping process, the steps of burning said waste liquors to form a smeltprimarily comprised of sodium sulfide and sodium carbonate, removing thegases formed during this heat treatment, contacting said smelt withcarbon dioxide and steam to convert sodium sulfide to sodium carbonate,maintaining said smelt in a substantially molten state during the abovestep, removing the gaseous products of reaction, and dissolving the thustreated smelt to form a solution primarily comprised of sodiumcarbonate.

4. A process as defined in claim 3, further including the step ofclarifying said solution.

5. In a process for treating sodium base waste liquors, the steps ofburning said liquors in a furnace to form a smelt primarily comprised ofsodium sulfide and sodium carbonate, passing said smelt to a reactor,contacting in said reactor said smelt with carbon dioxide and watervapor, maintaining the temperature of said contact above 851 C., passingthe gaseous products of the reaction formed in said reactor to saidfurnace, dissolving the thus treated smelt to form a solution primarilycomprised of sodium carbonate, and clarifying said solution.

6. A process as defined in claim 5, further including the steps ofburning said gaseous products in said furnace, and passing the gaseouscombustion products from said furnace to a sulfur dioxide recovery unit.

7. A process as defined in claim 6, further including the step ofpassing the gaseous combustion products from the furnace to a carbondioxide recovery unit.

8. A process as defined in claim 7, further including the step ofrecycling carbon dioxide recovered in the carbon dioxide recovery unitto said reactor.

9. A process as defined in claim 8, further including the step ofutilizing a portion of the clarified solution in the carbon dioxiderecovery unit.

10. In a process for recovering chemicals from sodium base wasteliquors, the steps of burning said liquors to form a smelt primarilycomprised of sodium carbonate and sodium sulfide, contacting said smeltwith carbon dioxide, steam and air, to convert sodium sulfide to sodiumcarbonate, maintaining the temperature of said conversion above 851 C.

11. In the recovery of chemicals from the soda base sulfite pulpingprocess wherein sodium base waste liquor is burned in a recovery furnaceforming smelt containing sodium sulfide, the sodium sulfide beingconverted to sodium carbonate which is treated to form sodium sulfite orbisulfite for use in the digesters, the combination with said soda basesulfite pulping process of the steps com prising:

(a) contacting said smelt containing sodium sulfide with hot steam andcarbon dioxide at a temperature above the melting point of sodiumcarbonate in a reaction zone thereby converting substantially all ofsaid sodium sulfide to molten sodium carbonate;

(b) taking hydrogen sulfide, residual carbon dioxide and steam overhead;

(0) contacting said hydrogen sulfide, residual carbon dioxide and steamin a furnace zone with combustion air to convert said hydrogen sulfideto sulfur dioxide;

(d) reacting said sulfur dioxide in a sulphiting zone with sodiumsulfite to form bisulfite;

(e) withdrawing said molten sodium carbonate from said reaction zone;

(f) introducing said molten sodium carbonate into a dissolving zone andthere dissolving said molten sodium carbonate in water to form anaqueous solution of sodium carbonate;

(g) employing a first portion of said aqueous solution of sodiumcarbonate as cooking liquor in the second stage of a two stagedelignification process, a second portion for reaction with sodiumbisulfite to form sodium sulfite for use in sulfur dioxide recovery andthe final portion for carbon dioxide make-up, the carbon dioxidereleased in the process being returned to said reaction zone.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN A PROCESS FOR TREATING SODIUJM BASE WASTE LIQUORS OBTAINED FROM AWOOD PULPING PROCESS, THE STEPS OF BURNING SAID WASTE LIQUORS TO FORM ASMELT CONTAINING SODIUM SULFIDE, CONTACTING THE RESULTING SMELT WITHCARBON DIOXIDE AND STEAM TO CONVERT SODIUM SULFIDE TO SODIUM CARBONATE,AND MAINTAINING THE TEMPERATURE OF SAID CONVERSION ABOVE 851*C.